中国科学院物理研究所
北京凝聚态物理国家研究中心
Q03组供稿
第49期
2020年06月19日
超导量子计算实验进展: 动力学相变的超导量子模拟

  shangshijiqishiniandai,wulixuejiafeimanwenyiweinianqingdetongshi: ruguogushenquyigeweizhidexianjing,erzhinengxiedaiyigerichanggongju,nidexuanzeshishenme?nianqingtongshidedaanshi: ruishijundao,erfeimanzijidexuanzeshi: jisuanqi!jiaoaorufeiman,yexuxiangdao,tahaishixuyaoyigexiaoxiaojisuanqi,cainengdulizhonggouxiandaikexuededasha。 buguohenkuaitajiugaibianlezhuyi,bashiniandaichu,feimanzhichu,jingdianjisuanjibingbushiyigejiejueliangziwentideyouxiaogongju,shijieshiliangzide,yigejiandandebaohanjishigelizideliangzixitong,jiuchaoyuelejingdianjisuanjidejisuanhecunchunengli,suoyimantangcaixuyaoyitailiangzijisuanji,yigeanzhaoliangzilixueyuanliyunxingdejisuanji。yuanguzhengjin,zaizhongguogudaiyongsuanpandeshiqi,renmenwufaxiangxiangjisuanjihuiyoushenmeyong,erxianzaimantangcaiyemianlintongyangdewenti,liangzijisuanjimantangcaixuyaoma?tanengzuoshenme?jingguoduonianyanjiu,renmenyijingtichulezhuduoliangzisuanfa,zhichuliangzijisuanjikeyijiejuehuoxunihuoxianshi、huoguoquhuoweilaidegezhongjingdianwenti。feimanshexiangdechangjingyexushisuizheliangzijishudefazhan,renmenjianghuipengdaoyuelaiyueduoxuyaojiejuedeliangzigongchengwenti,liangzijisuanjiwuyishijiejueliangziwentideyouxiaogongju。

  liyongyigekecaokongdeliangzixitongqumonifuzadeliangzixianxianghuozhejiejuejingdianjisuanjinanyijiejuedeliangziwentishiliangzijisuandezhongyaoyingyong。jinnianlai,suizhekejichengliangzibiteshudezengduo,xiangganshijiandeyanzhang,yijicaokongheduchujingdudetisheng,chaodaoliangzijisuanchengweiliangzimonidezhongyaopingtaizhiyi。yunyongjingdianjisuanjizhunquemoniliangziduotixitongdefeipinghengxingzhishirenmenzhangqiguanzhudeketi,eryunyongchaodaoliangzijisuanpingtaimonifeipinghengxingzhiweiyanjiuzheiyileiwentitigonglexintujing。 donglixuexiangbianzheiyigainianshifeipinghengdonglixuelingyuyizhiyilaideyanjiuredian,diyileidonglixuexiangbianguanzhufeipinghengxucanliang,dierleidonglixuexiangbianzeyushijianyushangluoshimitehuibo(loshmidt echo)defeijiexixingyijitongjiwulilideli-yanglingdiancunzaijinmilianxi,zuijindelilunyushuzhiyanjiubiaomingzheiliangleidonglixuexiangbiankeyinarutongyikuangjiajinxingyanjiu。

满堂彩  jinnianlai,zhongguokexueyuanwuliyanjiusuo/beijingningjutaiwuliguojiayanjiuzhongxinyuzhejiangdaxuezaichaodaoliangzijisuanyanjiufangmianhezuojinmi。jinqi,wulisuoxukaifuyanjiuyuan、boshishengsunzhenghang、zhengdongningyanjiuyuan、fanhengyanjiuyuanyuzhejiangdaxuewulixiboshishengliuwuxin、lihekangboshi、wanghaohuajiaoshoudeng,yijiribenlihuaxueyanjiusuozhangyuranboshi、yeli(f. nori)jiaoshouhezuotuandui,liyong16gechaodaoliangzibiteshixianledonglixuexiangbiandeliangzimoni,chengguoyujinrizaiguojixueshuqikan《science advances》fabiao。

  在此工作中,合作团队利用前期20量子比特薛定谔猫态工作中所研发的全联通超导多量子比特器件(图一),通过给超导量子比特施加一个同等振幅和相位的可控横场驱动,实现了Lipkin-Meshkov-Glick模型。该模型具有理论可预测的动力学相变现象,器件的多联通性质及驱动场调控可与模型及其参数相对应。实验首先展示了第一类动力学相变(图二)。在动力学铁磁相中(此时横场强度较弱),磁化率随时间的演化较为缓慢,磁化率的时间平均为有限值,破坏了Z2全局对称性;与之相反,在动力学顺磁相中(此时横场强度较强),磁化率的时间平均值为0且具有Z2满堂彩全局对称性。之后,通过测量洛施密特回波的时间演化,实验验证了在动力学顺磁相中存在洛施密特回波的零点,而在动力学铁磁相中,短时间内的洛施密特回波为有限值。上述实验结果间接揭示了这两类动力学相变之间的联系(图三)。最后,通过对多比特量子态自旋压缩性质的测量,揭示了动力学相变临界点和自旋压缩态产生的直接关联。通过测量不同横场强度下自旋压缩系数的时间演化,发现动力学相变临界点附近的自旋压缩最为显著,测得的压缩系数体现了多体真纠缠的存在,这一结果揭示动力学相变在量子精密测量领域的潜在应用(图四)。

  zhanwang: liangzimonixianzaiwanchengderenwuhaichuyuxianyoujisuanjidejisuanfanweizhinei,ershiyandezhuyaomudeyeshizhanshiliangzijisuanpingtaikeyianzhaorenmenyuqideliangzilixueyuanliyunxing,ersuizheliangzibiteshudezengzhang,jianglaideliangzimonijiangkeyiwanchengjingdianjisuanjisuobunengyucehejianyanderenwu,qizuoyongjiangbuketidai。

  cigongzuodedaoguojiazhongdianyanfajihua(no. 2017yfa0304300,no. 2016yfa0300600),zirankexuejijin(no. 11934018, no. 11725419,no. 11904393)yijizhongkeyuanbleixiandaozhuanxiang(no. xdb28000000)dengjijindezhichi。

满堂彩  benwendetongdenggongxianyizuowei: xukai(zhongkeyuanwulisuo)、sunzhenghang(zhongkeyuanwulisuo)、liuwuxin(zhejiangdaxue),tongxunzuozhewei: zhengdongning(zhongkeyuanwulisuo)、fanheng(zhongkeyuanwulisuo)、wanghaohua(zhejiangdaxue)。

参考文献:
Kai Xu#, Zheng-Hang Sun#, Wuxin Liu#, Yu-Ran Zhang, Hekang Li, Hang Dong, Wenhui Ren, Pengfei Zhang, Franco Nori, Dongning Zheng*, Heng Fan*, H. Wang*,
Probing dynamical phase transitions with a superconducting quantum simulator,
满堂彩 Science Advances 6, eaba4935 (2020).


图一: 左图为实验所用制备20量子比特薛定谔猫态时的器件概念图,这次实验选取了其中16个量子比特,这16个量子比特间的相互作用大小如右图所示。


图二: A磁化率的时间演化。横场强度为2MHz时,系统处于动力学铁磁相(DFP),而当横场强度为8MHz时,系统处于动力学顺磁相(DPP)。B磁化率的时间平均与横场强度的关联。实验结果与理论预测的动力学相变(DPT)点位置(由图中虚线标示)吻合。


图三: 洛施密特回波在动力学铁磁相和顺磁相中的时间演化行为。


图四: 时间域上自旋压缩系数的最小值随横场强度的变化。